Peripherally driven electroacoustical transducer



Dec. 26, 1950 J. J. ROOT 2,535,757

PERIPHERALLY DRIVEN ELECTROACOUSTICAL TRANSDUCER Filed June 25, 1946 2 Sheets-Sheet 1 l EN BY A T701? EY Dec. 26, 1950 J. J. ROOT 2,535,757

PERIFHERALLY DRIVEN ELECTROACOUSTICAL TRANSDUCER Filed June 25, 1946 2 Sheets-Sheet 2 Fan fi M l J7 6 5; 17

A ram/EV Patented Dec. 2 6, 1950 UNITED S TATES PAT ENT O F F vICE-1 2,535,757 rserpiieeminr nerves 'sLs'c'rRio- HCOUSTICHL TRANSDUCER .reiin-e..-nunr, New yor NQY. FxppIicationIJpne-ZB, 1946, Serial No. 6791100 8 laiil1's-- (Cl. I7- 9--1I5) The present invention "relates generally to elsetroacoustic transducers and it is more particularly directed to improvements iirl'oua speakers incorporating'inotors of the magnetic type.

The condenser type of speaker motor is one wherein mechanical forces result from electrostatic reactions. In effect, this mo'toris a "large condenser having. one flexible electrode free to move and act upon a diaphragm. The crystal type of speaker? motor is one whereinmechanical "L forces result from deformation of Ta crystalhaving converse piezoelecn-ic properties. since only small displacements. are possible without crystal fracture, mechanical levers are usedwhen moderate diaphragm excursions are'reqriired.

In magnetic speaker motors, the mechanical forces are derived from magnetic reactions This class includes'both moving coil "or electrooynamic speakers, andmagnetic armature'speakers. A moving. coil motor "is o'he'where'ih the mechanical forces result from the interaction between the field oi'the movingcoil and the constant polarizing field which it is disposed. The moving armature motor is one whose operation involves the vibration era ortion ore-rerroriragneti'c circult.

.In speakers incorporating meters of the above described types it"ha's liere'tbfofe beenthe general practice to impart the driving force from the motor to the center of the diaphragm and as a result these speakers have .been found to suffer from various. deficiencies. For example, in the electromagnetic speaker, the voice coil is customarily secured to the apex of a conical diaphragm.

Consequently the speaker cone is required to'be sufliciently massive and sturdy so as not to 'flex or. bend under the force exerted hy'the voice coil. This requirement adversely affects the speaker efficiency inasmuch as the quality of. lightness is requisite in a diaphragm inorder to attain sensi t'ivity and faithful high. frequency response. Similarly, inthemagne'tic, moving armature type motor, the motor is ordinarily coup'l'edthro'ugh a lever to the apex of a conical diaphragm. This type-of speaker is-alsosuhjectto resonance of'the driving lever which impairs the speaker tone quality.

Another important consideration in connection with loud speakers. is their overall physical dimension and weight. struction is such that its cone is apex driven, this acts as alimitlng factor inthereductionof speaker size. The reason for this is that the overall depth of the speaker assemblynotonl'y includes the depth of the cone but, in acldilfion,that of the Where the speaker condrive" motor, the motor usually being mounted be hind the apex of the cone. Moreover with conventional constructions, as the cone diameter is reduced, the low'fr'equency response of the speaker is diminished accordingly so that ncn linear' distortion is introduced as the speaker is made smaller.

In the view of the foi'egoinggit is the principal object of this invention to obviate theabove'de scribed limitations encountered in existing louid speakers by providing a "loudspeaker-of relatively small size and weight, characterized lay a high degree of sensitivity and uniform frequency response.

It is also an object ofthis invention 'to provide a loud speaker incorporating a magnetic motor wherein the speaker casing also functions as part of the ferromagnetic"circuit, thereby efiecting' a saving in speaker'weight and size.

, Still another object of this invention is to pro vide a loud speaker incorporating a magneticnio tor wherein a polarizing field is automatically established therein without 'r'eqiiiring a permanentmagnet ora separate field coil.

An additional object of this invention is to pro; vide a loud speakeras'senilily where overallceptn is substantially determined by the" depth of" its cone.

Still another object ofthis invention is to pro vide aloud speaker havingya'mininium ofcoinpm nents, of rugged construction, and adapted to simple assembly'and inexpensive manufacture.

For a complete understanding of this inven tic-n as well as other objects and features thereof"; reference is had to the following detailed description of several preferred emloodinients tobe read in connection with the accompanying drawing's. The scope of the invention will be pointed out in the accompanyingclaims.

In the drawings:

Fig. 1 is a top'plan view of a first preferre em bcdiment of a loud speakerin accordance with the principles of theinvention;

Fig- 2 is a vertical cross se'ctiontaken alongline 2- 2 of Fig. 1";

Fig. 3 1s a bottom plan view of said first embodii'ne'nt; V p

Fig. 4 is a view similar to Fig. 1 but partially broken away to illustrate various details of con struction; L

Fig; 5 is avertic'al section era-second preferred embodiment ofaloud'speakcr;

Fig. 6 is a schematic diagram-"illlistratlhg the application of a loud "speaker as in Fig. sto en electronic circuit;- v

Fig. 7 is a plan view of a modified energizing coil arrangement;

Fig. 8 is a vertical cross section of a third preferred embodiment of a loud speaker, and

Fig. 9 is a vertical cross section taken through line 9-4! in Fig. '7.

Referring now to the drawings and more particularly to Figs. 1 to 4, a preferred embodiment of a loud speaker is shown in various views, like numerals serving to identify like components in these figures. The speaker assembly is comprised mainly of an annular, cup-shaped base, generally designated by numeral [8, an annular cap adapted to cover said base, generally designated by numeral H, and a circular diaphragm interposed and supported between base It and cap ll, said diaphragm being generally designated by numeral [2. Base Ill in combination with cap I i also serve as the speaker casing and they are made of a ferromagnetic substance, such as steel. Diaphragm l2 is preferably fabricated of stiff paper or any other non-magnetic material having suitable acoustical properties.

Base Ill includes a reentrant column 53 and an outer cylindrical wall l4 having an outwardly extending flange l5, a shoulder i5a being formed on the border of the upper surface of the flange. Cap I l is formed with an annular plate 46 having an outer collar ll, said collar, in assembly, encompassing flange !5. A shoulder 16a is formed on plate [6 at the juncture of plate 16 and collar I1, shoulder l 6a being complementary to shoulder l5a. Cap I! is attached to base !B, such as by means of a plurality of machine screws l8 received in threaded bores in fiange i5.

Seated in the channel defined between column l3 and outer wall 14 is an energizing coil l9 wound about an annular insulating frame 26 which may be of Bakelite or similar material. A pair of leads 2i and 22 connected to energizing coil I9 are taken out through openings in base H] to serve as the terminals of the speaker.

Diaphragm l2 consists of a central cone 23 which may be partially corrugated as indicated and whose base is formed with a planar brim 2t connected to the cone 23 and having an oblique angular relationship therewith. In the assembled device, cone 23 is disposed within column 13 of the base. To support diaphragm l2, the border of brim 24 is bent downwardly and clamped between collar I l and fiange l5.

Secured on the surface of brim 24 of the diaphragm are a plurality of discrete armature strips 25 of magnetizable metal. Strips 25 are symmetrically and circumferentially arranged on brim 24. One modified form of diaphragm l2 involves imbedding strips 25 therein by disposing the strips between two layers of a laminated diaphragm material. The strips 25 may in another form be sprayed on the diaphragm surface.

H To operate the loud speaker from a vacuum tube amplifier, leads 2! and 22 are directly connected to the plate of the final power tube of the amplifier and the positive plate voltage terminal, respectively. It is desirable, of course, in order to obtain optimum speaker efficiency, to match the impedance of energizing coil l9 and the plate circuit of the power tube and this may be readily accomplished by proper design of coil impedance as by regulating the number of turns thereof.

' When no audio signal is applied to the power tube, the direct current component of plate current flow therein serves to energize coil l9, thereby polarizing base In and cap II and establishing a constant magnetic field therebetween. Upon the introduction of an audio signal to the amplifier, an alternating current component corresponding to the signal appears in the output of the power tube. This produces corresponding variations in the magnetic field strength of the speaker as the alternating component adds to and subtracts from the direct current component.

It will be evident that armature strips 25 are disposed within a small gap in the path of the magnetic lines of flux set up between base l0 and cap ll, annular plate It and the end face l3a of column 13 acting as opposin magnetic poles. It will further be apparent that the magnetic field assumes a circular pattern, the field being uniformly distributed with respect to the individual strips 25. Strips 25, being co-planar with plate It, tend to assume the same magnetic polarity as the plate and therefore are attracted in the direction of end face Be. The equilibrium position of diaphragm I2 is determined by the magneto field intensity in the steady state. As the field strength undergoes variation according to the applied audio signal, the position taken by armature strips 25 will concurrently shift therewith, thereby causing an axial vibration of diaphragm l2.

Shoulder 15a and shoulder l6a each serve to provide an air chamber on either side of diaphragm brim 24 which permits the excursion of the diaphragm from the equilibrium position, the air chambers also serving to dampen the diaphragm action. A plurality of circularly arranged ports 26 are formed in plate [6 which provide air columns communicating with the air chamber to prevent excessive vibration of diaphragm l2.

It is to be noted in the embodiment illustrated in Figs. 1 to 4, that diaphragm I2, by means of the action of the armature strips is driven at its peripheral area while its cone portion acts directly against the atmosphere. This technique has been found advantageous in that cone 23 is not subject to buckling with its attendant distortion such as results in an apex driven cone. Furthermore, there is no mechanical lever coupling between armature strips 25 and diaphragm 12, thereby eliminating the problem of objectionable leverresonance.

While the diameter of diaphragm l2 may be made very small, for example 3 inches, the low frequency response of the diaphragm is not seriously attenuated due to the relatively heavy mass inertia of the metal strips 25 around the peripheral area of the diaphragm. At the same time, however, the high frequency response is not materially affected due to the fact that the discrete strips 25 vibrate independently at high frequencies since the inertia of an individual strip is slight. Thus, a balance maybe established between the low and high frequency response of diaphragm 12. If it is intended to emphasize the low frequency range, strips 25 may be interconnected in whole or in part. The described construction further offers an opportunity to provide volume control means. I have determined that this can be done by enlarging the gap in which the armature strips 24 operate. This, of course, varies the reluctance of the circuit and the driving power accordingly.

Since a polarizing field is created when energizing coil is is directly connected to the plate output circuit of an amplifier tube, it is not essential to provide either a separate source of field potential or a permanent magnet. In

5: some instances, where audio signal momprment-iis purely alternating current without-1a direct current-superimposed: thereom :the same arrangement illustrated in Figs; ;1- to-t4 maybe employed except that armature strips lfirzare permanently magnetized. Another methodxo! providing a polarizing field is tohaveouterf-wall He permanently magnetized, the wall then? being preferably made of hard. steel or an alloy suitable. ior this purpose; The. remaining: of the' casingstructure may :beam'ade oi-f soft'iron which by contact with. outer wall: 14 become magnetized by induction "Thesewmethodsmay :benecessarwwhereithevoltage supplyioifthe amplifier 'is limiied 01-ld 0bt8iilfl2i stronger polariaing field, if required.

It is also some. observed thateby' driving diaphragm: 12 at itszperiphera-l area a. substantial reductionismade possiblaini'the space-occupied by the speaker assembly without a sacrificein.

speaker efliciency or. :quality.. In apex? idriven cone diaphragms, therspacesurrounding theicone 'is necessarily vacant, the --drivemotorwbemg ordi narilymountedtbehind theapex. As isiapparent, the: present invention. .iully' utilizes the" v:area surrounding the" cone: whereby the ioverall depth of the assembly hardly exceeds the cone. depth alone. This feature, in: addition 'tow :the ability "of diaphragm- 122 to afiordgood trequenoywre- :sponse: although of :small diameter, and :the use or :a casing which also tiunctions a wavportion oi Lthe ferromagnetic circuit, combines: to produce an effective miniature orphysically flat. speaker. It; can be seen. that the magnetic structnre is built away: aroma the. central. area of. the diaa phragm-so as to permit the ictormation or aiacone zthereat.

Another embodiment of". a: dot-id" speaker in ETC- cordance with the invention is illustrated in vertical crosswsectionin Fig.1 5 zand schematical connection with a push-pull amplifienin Eng; 16'. speaker operates in a push-pul-l manner; The advantages residing in the'push -pulltspeakcr arewzthe cancellation of second hamnon'icf distortion; therdevelopmentuoiz gr e ater' acoustical power and the elimination ofthe output transfomner of their push-pull amplifier with which zit-is elated; i a

The speaker assciriblzsr as shojwn in Fig: 57, comiprisesta base member' ZH identicaliwithx base =1! I in Fig. 2,. base 21 including a reentnant column '28 and an outer cylindrical wall 2 9 havi-ngi an outwardly extending flange: An energizing coil .30 is :seated in the-channel" defined bycol'umn 2B and outer-wall lfli Acap-member 31 is provided,

similar construction to base"?! exceptnthatsits outer wall 3-2 isextended and formed so as -"to encompass.- the *flange onwall w whenithe device iisassembled; An energizing coil-3fl is received rin the channel between outer wall 32 'and the'cy'lin- :drical column 31 :of cap 31". interposed between base 21 and cap 3| is .a diaphragm fii identical in construction with diaphragm I! of Fig. land including likesarmaiture strips.

Referring *now to Fig. 6, the fina-lpower stage of a push-pull amplifier =isshown; the medium eluding a. pair :of triotles'; so and 8 and apushzpull input transformer 38. The secondarywind :ing end terminals of transformer-3'8 "a;re c0n- *nected to respective :gridelectrodes of tribes- 3i and 31. One terminal of energi-zingcoirN is connected to pla tecf triode 36, the other terminal being connected to the positive plate voltage terminal 39.-

a like manner; one terminal of energizing coil 33 is connected to plate of triode 3 answer Gil, lihevother terminal heingicomiectedrto positive terminal- :38;. Whereas coils 3.1. :and. are each shown inJEi-g; .6. schematically as'having. twoi separate sections, they-\aneweach. inuactuality a continuous coil, the: separation being made to facilitatei-rthe schematic representation :of diaphragm 35.

Asuis characteristic of such push-pull behavior, triodes 36 and .3! are displaced. 180 degrees out of phasessin. operation, hence coils 31! and .33 are energizediin.opposinguphase relation.v A constant composite .magnetic'fieldis developed between the enduiace 28c ofcolumn. 28 and the end face 341; of :column 34; these faces serving :as: opposing magneticipoles which are in aiding relationship in respect to actuation of. the armature strips. The magnetic field intensity is modulated by audiosignals. The: armature strips are disposed intermediate end. faces 28a and 34a and are vibrated in. accordance: with the varying held to actuate :diaphragm. 35. It is to be noted that the polarity of themagneti'c; poles alternate as coils 3:! andBfiare alternately energized thereby providing. a. push-pull: diaphragm action. It will be: observed 'thatcthis construction eliminates the push-pull: output transformer in the amplifier which-generally has been. considered indispensable. In. the embodiment of. Figs. .1 to 4 the energizing coil to will be connected between the plate: of :a single output tube and the voltage supply in a manner similar to. the schematic representation ofzFigaG...

small speaker designs, that is, in those utiliz'ing: :diaphragms of less: than: five inches in diameter; the use. of a single energizin coil, as for example, coil 19 of Fig. 2, is fully satisfactory for developing a m-agnetomotive force ample to drive-theidiaphragm. (However; in large-speakers, becausewlarge coils have lessampere turns ratio, it. becomes desirable to .devise a coil arrangement for: the speaker enabling a greater ampere turns ratio within the same space taken by a single solenoid. One such. arrangement is. shown in planuview h Fig. 7 andin vertical section inIFig. 9. An annular base 411 of .magnetizablemetal is provided havingya cylindrical cut-er wall 4 I and a cylindricalinner wall 42. Intermediate Walls. 41 and-42' are-four, evenly spaced core. members '43.. 345 and integral with base. 49. Wound. about cores 43-.to 46 are. energizing: coils 47, 43, 4.9 and 511, respectively. The coils are all serially connected, input leads 5| and. 52 serving as. the speaker terminals; When coils 4'! 1:01.50 are. equally energized fromuan electrical source, base 431 becomescmagne-tically polarized, all of cores 4:3 to assuming one polarity and walls M and 42, the opposite polarity.

The magnetomotive force developed by this arrangement is greater than. obtainable with. a

single. coil occupying the. channel between walls a 4i and. The manner in which the magnetic fielddevelopedby base 40 is applied to an armature depends on the nature of the assembly design. It is obvious that any one of the polarized walls maybe extended to form a magnetic field of circular pattern in the manner indicated in Fig. '2 or 5; It is to be understood that while base-40 has been described as incorporating four energizing coils, the number thereof canbevaried as desired.

third preferredembodiment of a loud speaker isshownin Fig; 8. Anacoustica-l diaphragm 5'3is provided having atruncated conical form, the headportion =5! or thediaphragm being made Wreentrant to acoommodateamagnetic'motor; The

magnetic motor comprises an electromagnet 55 adapted to actuate a plurality of discrete arma-. ture strips 56 circumferentially arranged on the head surface 57 of the diaphragm.

Electromagnet 55 includes a cup-shaped base 58 of magnetizable metal, having a cylindrical outer wall 59 and a central post 63 about which is wound an energizing coil iii. Post 88 extends through a central opening in head portion and serves to support a magnetizable disc 62 having substantially the same diameter as outer wall 59. When electrical fluctuations are applied to energizing coil 6i, a magnetic field is set up between the end face 59a of outer wall .59 and disc.,62, the field assuming a. circular pattern. Armature strips 56 are uniformly distributed in the path of the magnetic iield and thereby cause axial vibration of diaphragm E3 in accordance with the applied electrical fluctuation. Diaphragm E3 may be corrugated, as indicated, and is supported about the periphery of the base of the cone by frame member 63.

Thus, there has been shown what at present are considered preferred embodiments of the invention. It will be obvious that many changes and modifications are possible therein without departing from the scope of the invention. For example, while the embodiments have. described diaphragms of circular shape, other shapes such as elliptical may be employed, if desired, in accordance with well known design practice to obtain emphasis of certain frequency bands. Therefore, it is intended in the following claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. An electroacoustic transducer comprising in combination a conical acoustical diaphragm, discrete and symmetrically spaced armature means disposed peripherally of said diaphragm, and magnetic means to actuate said armature means, said diaphragm being actuated by said armature means whereby sound waves are radiated from a central area of said diaphragm.

2. In a transducer device a diaphragm coniprising a nonmagnetic conical central area, a peripheral area, and a plurality of mutually spaced magnetic means on said peripheral area whereby magnetic action on said peripheral area drives said conical central area to produce sound radiations therefrom.

3. An electroacoustic transducer comprising in combination an acoustical diaphragm having a conical portion and an integral peripheral portion extending angularly from the base of said conical portion, a plurality of discrete ferromagnetic members disposed in symmetrically spaced relation on said peripheral portion, and magnetic means having an air gap formed therein, "said members being disposed in said air gap so as to serve as armatures therein, said magnetic means being adapted to have alternating currents of audio frequencies applied thereto whereby said armature members are actuated accordingly, said casing (serving to support said diaphragm at the outer edge of said peripheral portion, said casing having opposing pole faces. adjacent opposite. suffaces of said peripheral portion, an energizing coil responsive to electrical fluctuations for polarizing-said casing whereby a magnetic field lSieS'gtablished between said pole faces, the intensity, of said field corresponding to said electrical fluctuations, and a plurality of discrete armature strips symmetrically disposed on said peripheral portion so as to be withinsaid magnetic field, the axial position of said armature strips being controlled by the intensity of saidmagnetic field whereby said diaphragm is actuated in anaxial direction in response to an audio fluctuating magnetic field to produce sound radiation.

5. An electroacoustic transducer comprising in combination an; acoustical diaphragm having a central conical portion and a peripheral portion extending integrally from the base of said .cen

tral portion, a magnetic motor for actuating said diaphragm,said motor comprising a ferromagnetic casing havingannulafly arranged opposite poles adjacent opposite surfaces of said peripheral portion, an energizing coil adapted to be actuated by an alternating current superimposed on a direct current whereby said coil may be connected between the plate of an electronic tube and the direct current voltage supply therefor so as to produce a varying magnetic field between said poles in response to variations ,in th selectron stream of said tube, and a plurality 0f,spaced armature strips radially arranged on said peripheral portion so as to be between said poles and within said magnetic field, said armature strips being thus adapted to be actuated axially by said varying magnetic field so as to radiate acoustical energy in response thereto.

6. An electrical transducer according to claim 5 wherein said, diaphragm is of non-metallicma- ,terial, said armature strips being discrete, metallic members and being imbedded in said nonmetallic diaphragm. v

'7. An electroacoustic transducer comprising'in combination a conical acoustical diaphragm, .a

plurality of discrete ferromagnetic membersdisposed peripherally of said diaphragm, and magnetic means to actuate said members, said magnetic means being adapted to have a direct current superimposed with an alternating .current applied thereto whereby said members are driven according to said alternating current said diaphragm being actuated by saidwdriven members whereby sound radiation is produced therefrom. 8. In an electroacoustic transducer, an acoustical diaphragm including a conical portion and a peripheral portion extending integrally from the base of said conical portion, and a plurality of discrete metallic strip afilxed to the peripheral portion at symmetrically spaced positions and-annularly arranged thereon, said strips being adapted to be actuated upon by a varying magnetic field so as to produce corresponding actuation of said conical portion, the mass inertia of said strips being such as to render said diaphragm responsive to a low audio frequency range, the individual inertia of such strips being such as to .enable each to vibrate independently in a higher audio frequency range whereby both low and high audio frequencies may be reproduced b said diaphragm when actuated.

JOHN J. ROOT.

(References on following page) REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,613,609 Harrison Jan. 11, 1927 1,636,410 Frederick July 19, 1927 1,795,948 High Mar. 10, 1931 Number 10 Country Date ott Aug. 22, 1939 Thuras Sept. 5, 1939 Price Dec. 3, 1940 Mott July 15, 1941 Wells Feb. 24, 1942 Marguis et a1. June 24, 1947 Mott May 9, 1950 Wirsching May 9, 1950 

